1. Field of the Invention
The present invention relates to a porous honeycomb structure body, the use thereof and a method for manufacturing the same. More specifically, it relates to a porous honeycomb structure body having a reduced pressure loss, with maintaining isostatic strength, by control of distributions of porosity and a pore diameter of the structure body, the use thereof and to a method for manufacturing the structure body. The porous honeycomb structure body of the present invention can be preferably used particularly as a particulate filter for exhaust gas purification, and a carrier for catalyst.
2. Description of the Related Art
Recently, the influence of particulate matter and NOx discharged from an automobile engine, especially a diesel engine or the like on environment has been increasingly receiving attention. Therefore, various research and developmental works have been made on the use of porous honeycomb structure body as important means for collecting and removing such deleterious substances.
For example, a honeycomb structure body is under developing to collect and remove particulate materials in exhaust gas by making exhaust gas flow into individual through holes having openings at one of the end faces of the honeycomb structure body comprising a plurality of through holes which are partitioned by porous partition walls and whose openings on an end face where an exhaust gas flows in and an end face where an exhaust gas flows out are alternately sealed, and passing it forcibly the partition walls thereof. A developmental work is also in progress, as a new approach to improve purification ability of such substances, to provide a catalyst made of a honeycomb structure body being composed of a porous structure all of which partition walls have a high porosity to which catalysts for decomposing HC and NOx are loaded in a relatively larger quantity.
Upon use of such a porous honeycomb structure body, it is housed in a metal case or the like by means of a holder with a fixed pressing force so that the structure body would not be displaced from the metal case under continuous vibrations from an engine or the like. Therefore, the structure body is required to have isostatic strength that enables the structure body to endure the pressing force. Especially, an attempt has been made to make the porosity in a honeycomb structure body higher, in responding to the demands in the reduction in pressure loss for less fuel consumption and high output, or the demands of the increase in the loaded amount of the catalysts for improvement of purification ability. Accordingly, it is strongly desired to have a honeycomb structure body provided with a sufficient isostatic strength with satisfying the demands for making the porosity in the honeycomb structure body higher. Incidentally, in the case of a honeycomb structure body installed in the path for exhaust gas, the flow amount of the exhaust gas in the center portion of the honeycomb structure body along with the perpendicular direction of the path for exhaust gas. Thus, the easiness in the flowability of exhaust gas and the decomposed amount of the HC, NOx or the like in the center portion of the honeycomb structure body awfully give influence on overall pressure loss and the purification ability. As a consequence, the development of the structure bodies and catalyst bodies having a structure capable of copping with the differences in the distribution in flowing gases is earnestly desired.
There has been proposed as a prior art porous honeycomb structure body to attain the objectives mentioned above a porous honeycomb structure body having a prolonged durability time for collection of particulate materials with a reduced frequency of regeneration treatment by constituting the honeycomb structure body to have xe2x80x9ca porosity of 45 to 60%, pores with a pore diameter of 100 xcexcm or more in a volume that corresponds to 10% or less of a total volume of all pores, and a relationship between a total of specific surfaces (Mm2/g) of all pores opening on the surface of the structure body and penetrating the structure body inwardly and surface roughness (Nxcexcm) on the surface of the structure body of 1,000M+85Nxe2x89xa7530xe2x80x9d (See Japanese Patent No. 2726616).
Further, there is disclosed a porous ceramic honeycomb structure body having significantly prolonged collection time, with the same collection efficiency and the same pressure loss, by having xe2x80x9ca porosity of 40 to 55% and a total volume of pores having a diameter of 2 xcexcm or less being 0.015 cc/g or lessxe2x80x9d (See Japanese Patent No. 2578176).
In addition, a cordierite honeycomb structure having a high collection rate, a small pressure loss and a small coefficient of thermal expansion simultaneously by having xe2x80x9ca thermal expansion coefficient between 25xc2x0 C. and 800xc2x0 C. of 0.3xc3x9710xe2x88x926/xc2x0 C. or less, a porosity of 55 to 80%, an average pore diameter of 25 to 40 xcexcm, and smalls pores each having a diameter of 5 to 40 xcexcm and large pores each having a diameter of 40 to 100 xcexcm as pores on surfaces of partition walls, the number of the small pores being 5 to 40 times as large as the number of the large poresxe2x80x9d is also disclosed (JP-A-9-77573).
However, with respect to any of these honeycomb structure bodies, it has been never given any consideration to simultaneous satisfaction of such a characteristic as the reduction of pressure loss and the attainment in increased isostatic strength which are mutually contradictory one by controlling the pore distribution.
The present invention has been completed in view of the above problems. An object of the present invention is to provide a porous honeycomb structure body capable of satisfying simultaneously the characteristics of pressure loss and isostatic strength which are mutually contradictory properties, suitable for use particularly in an exhaust gas purification device installed in combustion equipment and which is usable, for example, for a structure body for collecting and removing particulate substances contained in exhaust gas or a catalyst body for decomposing HC, NOx and the like to remove them therefrom, a method for manufacturing the porous honeycomb structure body as well.
The intensive studies have been made so as to solve the above problems. As a result, firstly, we have found the phenomenon that the firing shrinkage of a honeycomb molded article containing a cordierite-forming raw material as a major component and carbon as a pore-forming agent becomes extremely evident when the product temperature of the molded article is within a temperature of 1,000 to 1,200xc2x0 C., but the firing shrinkage of the molded article hardly occurs when the temperature of the molded article is outside the above-mentioned temperature range of 1,000 to 1,200xc2x0 C.
Then, we have made further studies and found that a honeycomb structure body having a large pore diameter and porosity in a center portion which greatly influences a reduction in pressure loss can be obtained by using carbon as a pore-forming agent and controlling a temperature increasing rate of a firing environment so as to prevent carbon existing in the center portion of a molded article from burning out until the center portion of the molded article exceeds the above-mentioned temperature range. The present invention has been completed based on the present finding.
That is, according to the present invention, there is provided a porous honeycomb structure body having partition walls which contain cordierite as a primary crystal phase, a porosity of 40 to 75% and an average pore diameter of 10 to 50 xcexcm, wherein the porosity and the pore diameter in a center portion of the honeycomb structure body are larger than those in a peripheral portion of the honeycomb structure body. In the present specification, the phrase xe2x80x9ccenter portionxe2x80x9d refers to a midpoint of a central axis of a honeycomb structure body or molded article or a partition wall portion which is the closest to the midpoint, while the phrase xe2x80x9cperipheral portionxe2x80x9d refers to the outermost partition wall portion from a midpoint of a central axis of a honeycomb structure body or molded article in a direction perpendicular to the central axis. In the honeycomb structure body of the present invention, the porosity and the pore diameter are defined with respect to the xe2x80x9ccenter portionxe2x80x9d. However, an area having a larger porosity and a larger pore diameter than the peripheral portion may possess a certain spread area from the center portion. Additionally, in the present specification, the terms xe2x80x9cporosityxe2x80x9d and xe2x80x9cpore diameterxe2x80x9d mean the average porosity and average pore diameter, unless they are specified.
In the present invention, the porosity in the center portion of the honeycomb structure body is preferably larger than the porosity in the peripheral portion of the honeycomb structure body by 2% or higher, more preferably by 3% or higher, and the pore diameter in the center portion of the honeycomb structure body is preferably larger than the pore diameter in the peripheral portion of the honeycomb structure body by 2 xcexcm or larger, more preferably by 3 xcexcm or larger.
Further, according to the present invention, there is also provided a method for manufacturing a porous honeycomb structure body which comprises the steps of preparing a molded article having a honeycomb structure by use of a puddle containing a cordierite-forming raw material as a primary raw material and carbon in an amount of at least 5 parts by mass based on 100 parts by mass of the cordierite-forming raw material and drying and firing the obtained molded article, wherein upon firing of the molded article, temperature of a firing environment is increased at a rate at which carbon existing in a center portion of the molded article is burned out within a temperature range of from 1,200xc2x0 C. to below 1,430xc2x0 C. in terms of the temperature of the center portion of the molded article.
In the present invention, it is preferable to increase the temperature of firing environment, although depending on the type of carbon to be used, ordinarily at a rate of 20 to 60xc2x0 C./hr within a temperature range of 400 to 1,150xc2x0 C.
Further, the temperature of the firing environment is preferably maintained within a temperature range of 1,150 to 1,200xc2x0 C. for at least 5 hours after the temperature reaches at 1,150xc2x0 C.
In addition, in the method for manufacturing of the present invention, the molded article having a honeycomb structure is preferably manufactured by use of a puddle containing at least carbon in an amount of 25 parts or less by mass based on 100 parts by mass of the cordierite-forming raw material. Furthermore, the molded article having a honeycomb structure is more preferably manufactured by use of a puddle containing a formable resin in an amount of 5 parts by mass or less based on 100 parts by mass of the cordierite-forming raw material. Moreover, the atmosphere in the firing furnace in which the molded article is fired is preferably set at an oxygen concentration of 7 to 17% by volume at least when temperature of firing environment is from 400 to 1,150xc2x0 C.
Next, with reference to FIG. 1, a relationship between a temperature increasing rate and pore formation in a firing step in the method for manufacturing of the present invention will be described. FIG. 1 is a graph illustratively showing the state of the temperature of a firing environment and the temperature of a center portion of a honeycomb structure body in a firing step in one embodiment of the present invention. In FIG. 1, a solid line represents temperature of the center portion of the molded article, and dotted lines represent temperature of the firing environment. Further, the temperatures of the center portion of the molded article were measured by inserting an R-thermocouple into a through hole and setting the R-thermocouple in the center portion of the molded article.
As shown in FIG. 1 showing an example wherein a graphite is used as a carbon source, in the method for manufacturing of the present invention, when temperature of firing environment reaches a temperature at which carbon contained as a pore-forming agent can burn, i.e., about 600xc2x0 C. in FIG. 1, temperature of a center portion of a molded article becomes higher than the environment temperature. This indicates that the carbon contained as a pore-forming agent started to burn and the temperature inside the molded article was increased thereby. When the temperature of the molded article reaches 1,000 to 1,200xc2x0 C. by further increasing the environment temperature, firing shrinkage of the molded article comprising a cordierite-forming raw material becomes the most noticeable, although such a phenomenon did not become apparent in the graph.
At this point, if carbon has been already burned out and pores are already formed in the honeycomb structure, the pore diameter formed in the honeycomb structure is shrunk due to the firing shrinkage. If the carbon still remains as in the example shown in FIG. 1, however, firing proceeds with maintaining the diameters of the pores at the diameters of carbon. Further, when temperature of the molded article exceeds 1,200xc2x0 C. with increasing the firing environment temperature, the firing shrinkage of the molded article becomes small. Moreover, if carbon is completely burned out at this point, pores substantially equal to the diameters being originally possessed by carbon are formed. Hence, the pores thus formed may have larger diameters, compared with the diameter of the pores having been formed as shrunk one due to the burning out of carbon below temperature of 1,000xc2x0 C.
When carbon in the molded article is burned out in a certain portion, temperature of that portion of the molded article sharply decreases to the environment temperature or below. Namely, if that portion is the central portion of the molded article, one may observe the occurrence of a peak at temperature from 1,200 to 1,300xc2x0 C., as is shown in FIG. 1. That is, if an inner temperature of the molded article, at which such a sharp temperature change as mentioned above occurs, overlaps 1,000 to 1,200xc2x0 C. of the environment temperature in which the firing shrinkage of the molded article is observed the most noticeably, the firing shrinkage is enhanced. Resultantly, tears in the structure are observed due to firing shrinkage.
Therefore, in the present invention, since the above-mentioned peak temperature, i.e., the inner temperature of the molded article at the time when carbon being present in the center portion of the molded article is burned out, is controlled so as to exceed 1,200xc2x0 C. by controlling the temperature increasing rate of the firing environment, pores of the honeycomb structure being present in the center portion thereof may have a diameter equal to that of carbon having been present in the center portion, without causing tears due to firing shrinkage.
Further, carbon present in a peripheral portion of the molded article; said portion being in a more aerobic environment is burned out more easily than carbon present in a central portion including the center portion and is often burned out at a relatively low temperature and forms pores. Thus, diameters thereof are often shrunk due to subsequent firing shrinkage. Accordingly, there are often observed the difference in the porosity and the pore diameter between the central portion and the peripheral portion.